For p,q,r,s,t∈Z+ with rt≤p and st≤q, let G=G(p,q;r,s;t) be the bipartite graph with partite sets U={u1,…,up} and V={v1,…,vq} such that any two edges ui and vj are not adjacent if and only if there exists a positive integer k with 1≤k≤t such that (k−1)r+1≤i≤kr and (k−1)s+1≤j≤ks. Under these circumstances, Chen et al. (Linear Algebra Appl. 432:606-614, 2010) presented the following conjecture: